This invention relates to air conditioning evaporators in general, and specifically to an improved air fin design that enhances the drainage of condensate.
Automotive air conditioning system evaporators are subject to water condensate formation, by virtue of being cold and having humid warm air blown almost continually over them. Water condenses on the tube or plate outer surfaces and fins, partially blocking air flow, increasing thermal resistance, and potentially even shedding or xe2x80x9cspittingxe2x80x9d liquid water into the ductwork of the system. A screen is often installed downstream of the evaporator to block water shedding, adding considerable expense.
To the extent that condensed water can be forced or encouraged to drain down and out of the evaporator, the above noted problems are reduced. Some obvious and low cost expedients include orienting the evaporator core so that the flat outer plate or tube surfaces are oriented vertically (or nearly so), with open spaces between them at the bottom of the core, so that downward drainage is assisted, and at least, not blocked. Vertical troughs or channels have been formed in the outer plate surfaces, as well, for the same reason.
An inherent problem with vertical plate or tube orientation is that it creates a resultant air fin orientation that is not conducive to condensate drainage. That is, the corrugated fins brazed between the flat plate surfaces are given a nearly horizontal orientation when the plates are arranged vertically, thereby acting as dams to block drainage flow down the plate surfaces. Numerous fin designs have been proposed with notches cut through, or stamped into, the fin corrugation peaks or crests, to thereby provide drains through the fins. Such designs would be considerably more difficult to manufacture, and also remove substantial contact area between the fin crest and plate surface, reducing thermal conduction efficiency between the two.
Fins also typically include banks of thin, angled louvers cut through the fin walls, oriented perpendicular to the air flow, which are intended to break up laminar flow in the air stream, enhancing thermal transfer between the fin wall and the air stream. Louvers are invariably arranged in sets of oppositely sloped pairs or banks, so that the first louver pattern will turn the air stream in one direction, and the next will turn it in the other direction, for an overall sinuous flow pattern. The cutting of the louvers inevitably leaves narrow gaps through the fin walls through which condensate can drain, under the proper conditions.
At least one prior art design claims a connection between the louvers and condensate handling. U.S. Pat. No. 4,580,624 simply proposes to assure that the last, most downstream pattern of louvers on the fin wall be sloped inwardly, toward the interior of the core, rather than sloped toward the exterior. It is claimed that this orientation causes condensate drainage at this downstream point to also flow inward, rather than being blown out into the duct. This is a somewhat odd claim, especially since, with the essentially universal louver pattern of oppositely sloped pairs or banks, the most downstream louvers would be sloped inwardly, anyway, and would inherently do what is claimed. Moreover, a fast air stream moving up through the most downstream louver bank could overwhelm the drainage force, shedding the water regardless, unless the last louver pattern were very steeply sloped. It would be essentially impossible to manufacture a fin in which only the most downstream louver bank was steeply sloped, and putting a very steep louver angle on all louvers in the fin would increase the air side pressure drop considerably.
Another apparent trend in evaporator air fins is the use of corrugated fins in which the fin walls are oriented parallel to each other (or nearly so), in a U shaped corrugation, or in a shallow V with a relatively large radiused crest, rather than a sharper crested V. At least part of the impetus for this trend is the desire for a dense fin pattern or fin pitch, one that puts more fin walls per unit length within the available volume. A wider V shape, in general, would create a less dense pattern of fewer fin walls per unit length, at least for a given radius of the crest. Furthermore, a more rounded, less sharply radiused corrugation crest would be considered desirable in that it provides the only surface area of the fin that directly contacts the plate or tube outer surface. A corrugation crest with a smaller radius (a sharper xe2x80x9cVxe2x80x9d) would provide less mutual contact area. While denser fin patterns theoretically provide more fin-to-air-stream contact, and more fin-to-plate mutual surface contact, which would increase thermal efficiency, the effect on condensate retention has apparently not been closely considered.
An example of an evaporator fin design with parallel walls, and large radiused or U-shaped crests joining the fin walls, is disclosed in U.S. Pat. No. 4,892,143. The design claims lower condensate retention, but claims that such a result is due to a factor that is very much at odds with the actual operation of an evaporator fin of that type, as described further below. The patent claims that by reducing the unlouvered length of the outside of the fin wall and holding it within a small range, that the amount of condensate xe2x80x9ctrappedxe2x80x9d on the exterior of the crest between adjacent fin walls is reduced. In point of fact, with a fin of this design, it is found that water condensate is strongly retained between the facing inner surfaces of the fin walls, on the interior of a fin corrugation, but not on the exterior of the fin crest to any significant extent. It may have been assumed, from observation, that where condensate was not seen, it was somehow being drained or removed, when in fact it had simply not formed in the first instance. In actuality, fin shape design disclosed in the patent, with parallel fin walls and large radiused, U-shaped crests, is the worst performing in terms of retained condensate.
The invention provides an evaporator with a fin pattern that provides enhanced drainage of water condensate from between the fin walls and out of the evaporator, without degrading the performance of the evaporator otherwise.
In the embodiment disclosed, a laminated type evaporator has a series of spaced tubes, the opposed surfaces of which are separated by a predetermined distance. A corrugated air fin located in the space between opposed plate surfaces is comprised of a series of corrugations, made up of a pair of adjacent fin walls joined at a radiused crest. Each fin wall is pierced by a louver, the length of which is determined by that portion of fin wall not taken up by the radiused crest. Adjacent crests joining adjacent pairs of fin walls are separated by a characteristic spacing or pitch, with smaller pitches yielding higher fin densities, and vice versa. For a given pitch and tube spacing, a volume or cell is defined between the tube surfaces within which each corrugation (pair of fin walls and crest) is located.
According to the invention, the shape of the corrugation within that cell, in terms of radius and relative louver length, is determined and optimized as a function of a series of defined ranges of the ratios of fin pitch, louver length, and crest radius, all to plate spacing. Based on a combination of empirical testing and computer modeling, optimal ranges of those parameters that determine corrugation shape have been determined, as a function of tube spacing, and based on practical considerations of desirable heat flow performance, air pressure drop through the fin, and water retention on and in the fin. For a given tube spacing, the designer can choose a corrugation shape (crest interior radius, fin pitch, and louver length) that will improve condensate drainage significantly, while not significantly degrading the evaporator performance in other areas.